Conventionally, systems that transmit and receive data via communication between communication devices have been considered. In such systems, ensuring security when performing data communication between the communication devices is essential. Especially, for instance, when communicating via an open communication channel that is accessible from the outside, possibility of data leakage, tampering, and attacks including spoofing increases.
Regarding such a problem, in order to prevent replay attacks, in which an attacker spoofs the transmitter by directly reusing captured encryption data, conventional technology (for instance, Patent Literature 1) discloses a system in which both a communication device of a transmitting side and a communication device of a receiving side utilize count values. In the system, the communication device of the receiving side confirms synchronization of the communication devices by comparing a count value stored therein and a count value received from the communication device of the transmitting side.